3.3 Gascoyne River catchment condition summary
3.3.4 Summation
A significant problem within the wash plains of the catchment, as well as elsewhere, is the disruption to surface hydrology by infrastructure (e.g. roads, tracks, fencelines) (Figure 62).
Where vegetation cover is drastically reduced infrastructure initiated erosion problems have a considerable impact on general rangeland condition. This problem is illustrated in the sequence of photographs shown in the section 3.2.2.3 (Erosion exacerbated by infrastructure—Figures 36 & 37a–c).
Figure 61 Fragmenting vegetation banding, Jamindie land system; Gascoyne River catchment (2006 aerial photograph provided by Landgate). Inset: Foreground shows stripped interpatch which progressively becomes covered by mobilised sediments eroding from the rear of degrading grove
Figure 62 Water-starved gidgee grove downslope of a road
Being downslope of run-off areas increases the potential for wanderrie banks to produce useful pasture, especially when able to retain run-on (Figure 63). However, overgrazing has reduced the perennial grasses to such an extent that the low strata of many sandy banks now only supports annual grasses such as wind grass (Aristida contorta) and annual wanderrie grass (Eriachne aristidea) (Figures 64a, b).
Figure 64a, b Wanderrie banks reduced to supporting only wind grass (Aristida contorta) and annual wanderrie grass (Eriachne aristidea)
Figure 63 Wanderrie banks in fair condition supporting some perennial grasses; Buck wanderrie grass (Eriachne helmsii)
Riparian pasture productivity is highly variable. Initial settlement of the Gascoyne catchment was along the river, with stock reliant on river pools and natural springs. Consequently, many riparian pastures are overgrazed and degraded. Where buffel grass is well
established, it has a significant role in stabilising surfaces and preventing further erosion. In addition, buffel grass colonisation has increased the productivity of some riparian pastures in favourable seasons. However, stock numbers in favourable seasons are often above that which the surrounding native vegetation can support in the absence of buffel grass (Wilcox
& McKinnon 1972; Payne, Curry & Spencer 1987) and this leads to areas of overgrazing.
With the onset of dry conditions the protein content of buffel grass declines and livestock seek supplementary forage. Stock migrate upslope and fertile patches become the primary browse source. Without appropriate stocking rates, fertile patches are overgrazed, leading to deterioration as a forage source but also their capacity to retain water. This reduces their resource capture role and contributes to escalating erosion downslope.
In the catchment’s lower reaches, saline alluvial plains are one of the more dominant land types, the other being sandplains. Many of the land systems within these land types occur within the Lower Gascoyne Alluvial Plains Zone of the Carnarvon Province. Here the floodplains, levee sand banks and adjacent alluvial plains are dominated by buffel grass.
However, such buffel grass dominated plant communities are now increasingly susceptible to fire following good seasons. Fire sensitive species, such as those belonging to the Chenopodiaceae, may disappear. Whilst buffel grass can re-establish, such landscapes become inherently fire prone and therefore susceptible to future periods with exposed surfaces prior to post-fire recolonisation.
4 Discussion
The Gascoyne River catchment is in poor condition (characterised by loss of cover, few perennial plants and ongoing soil loss), with many areas continuing to decline. The poor condition of the catchment is not a recent issue. Previous reports have established that the condition of the Gascoyne River catchment was in poor condition (Wilcox & McKinnon 1972;
Jennings et al. 1979; Williams, Suijdendorp & Wilcox 1980; House et al. 1991; Hopkins, Pringle & Tinley 2006). Many of the areas in poor condition were likely to have been so since the 1930s or earlier (Williams, Suijdendorp & Wilcox 1980).
Within the catchment there has been a 15% decline in the number of perennial shrubs in the last five years (a 39% decline from the perennial plant numbers recorded in the above average seasons of 1995 to 2000), reduced resource capture (13% decline overall and 22% decline in groves) and an increase in erosion features (Section 2.2.2). Over 3.6 million hectares were assessed as being in poor condition for the years 2002 to 2009 (Section 2.2.2.1). The overall trend in vegetation cover (1989 to 2010) was stable, thus areas that were in poor condition are still in poor condition. The practice of continuous stocking through consecutive dry years (Annual Return of Livestock and Improvement forms, Pastoral Lands Board (PLB)), in excess of the carrying capacity of the resource (Wilcox &
McKinnon 1972; Payne, Curry & Spencer 1987), has contributed to the poor condition of the catchment.
Large contiguous areas are declining in perennial vegetation cover in the catchment between the central Gascoyne and Lyons rivers. Plant numbers in 2011 at monitoring sites (WARMS) had declined to 1995 levels. In particular, satellite images indicate that the seasonal
conditions for large areas of the central Gascoyne and lower Lyons rivers had poor seasons in four or more years prior to the December 2010 flood. The greenness index at the time of the flood was low and as a consequence the groundcover is likely to have also been low.
Vegetation, groundcover and obstructions are fundamental to sheet flow and erosion control (Coles & Moore 1968; Tongway & Ludwig 1996, 1997). However, it is difficult to determine to what degree catchment condition and groundcover contributed to the Gascoyne River 2010–11 summer floods.
The spatial arrangement throughout the catchment of sparse capture zones (patches) interspersed between long interpatches allows water energy to increase during run-off.
Whilst capture zones have higher infiltration rates (Section 2.2.3) they are, in general, a relatively small component of the landscape (ratio of interpatch to patch estimated at 88:12). Increasing the number of capture zones through the number of plants or fallen timber obstructions would increase infiltration capacity over time. However, as the number of obstructions has declined erosion has increased in both shedding and capture zones.
Fewer resources are being retained in the landscape as infiltration areas decline in size and quantity, as indicated by the reduction in RCI between 2006 and 2011. Clearly, the high ratio and extent of interpatches results in rapid watershed and would contribute significantly to flooding through the catchment.
However, the magnitude of the December rainfall event, in excess of 200 to 300 mm of rainfall over a 24-hour period, was such that the subsurface and surface storage capacities of the soil would have been exceeded irrespective of infiltration rates on the interpatches.
Where soil profiles were described at the WARMS sites, they were frequently less than 30 cm deep and often consisted of a sandy loam over clay, hardpan or weathered rock.
This was particularly common on the interpatches where hardpan was encountered.
Assuming a maximum storage capacity of 20% gives total soil water storage of 60 mm, the December rainfall event exceeded this amount by at least three to five times when the
profiles would have been dry. With subsequent rains, the soils would have already been moist and therefore had less storage capacity. It is therefore likely that many soils would have reached their storage capacity within a few hours of the January and February 2011 events.
As discussed in Section 2.2.1, the hydrographs for major floods since 1960s have been of similar magnitude and shape. It suggests that the catchment characteristics including catchment condition have changed little during this period; as mentioned above it is likely catchment condition has been poor at least since the 1960s. However, from the current analysis it is not possible to assess the impact catchment condition on the magnitude of the December 2010 flood.
Erosion from the December 2010 flood was large by comparison to other major flood events, based on estimates of the size of the sediment plume, sediment loads in the plume and observation of the Gascoyne River channel. However, the rainfall event was so exceptional that, as with the flooding, it is highly likely that the river channel would have experienced some erosion regardless of the condition of the catchment. Erosional features, assessed at
WARMS sites (Section 2.2.2.2, Tables 5 and 6) and described in Section 3, are widespread throughout the catchment and have increased over the monitoring period. It is not always possible to attribute these features to the floods in the summer of 2010–11, but it is almost certain that these features have developed as a result of loss of groundcover since European settlement. Gully head migration upslope and straightening of drainage tracts, allowing faster drainage, are visible processes. Vegetative groundcover reduces erosion, and it is clear that erosion would be much less if the catchment was in better condition.
The catchment is naturally a high watershedding catchment. The loss of vegetative cover is causing accelerated erosion and the area within the catchment that sheds water has increased. This has significantly reduced the capacity of the land to retain resources. The Gascoyne River catchment continues to dry out and erode. Vegetation is increasingly
dependent on in-situ rainfall, rather than run-on, and larger rainfall events are required to flood incised drainage tracts and return water to water-starved plains. The reduction in soil
moisture balance increasingly favours plant species adapted to desiccating soil profiles and growth becomes increasingly episodic as run-off increases and deep soil moisture storage declines. The desiccation process will continue until new base levels are reached, resulting in water ponding, deposition and soil accumulation in equilibrium with erosive processes.
Natural recovery in arid and semi-arid shrublands is slow. To reduce the flood impact from large rainfall events vegetation groundcover and obstructions need to be increased. There was a substantial increase in shrub numbers between 1995 and 2000 in response to above average seasons across the catchment. However, new plant recruits require time to
establish and develop as capture zones, with time increasing litter accumulation and, eventually, infiltration rate. Overlaying a pastoral operation on this regenerative process significantly adds to the challenges of vegetation recovery in a rangeland environment. As well as time, appropriate management strategies are crucial to the success of attempting to reverse the dysfunctional processes within the Gascoyne River catchment.
With many upper slopes, interfluves and drainage flats severely degraded the carrying capacity of the Gascoyne River catchment has significantly diminished. Analysis of WARMS site data indicates that the density of palatable perennial shrubs has declined (Section 2.2.2.2.1). Present day pastoral operations are increasingly reliant on riparian pastures, especially where drainage margins have become colonised by buffel grass which provide abundant forage in favourable seasons. However, stocking rates based on good season riparian pastures often exceed the carrying capacity of the rest of the impoverished landscape.
Secondly, with the increased fire susceptibility of buffel grass pastures, wildfire will significantly damage remnant vegetation communities and leave soil surfaces further exposed. Should a flood event follow a wildfire, the watershed across the burnt, bared surfaces, lacking in obstructions can only result in flooding and increased erosion.
Many vegetation communities throughout the catchment are under stress due to escalating catchment dysfunction, resulting in widespread erosion and desiccation. With the onset of dry conditions, the few remaining fertile patches (Section 2.2.2.2.2) receive increased grazing pressure as stock search for additional forage to supplement the nutrition formerly provided by seasonally dependent plants. This has resulted in overgrazing of favoured sites, especially prior to adjusting stocking rates to the changed conditions. Reported stock numbers in the catchment, as supplied by lessees to the PLB through Annual Return of Livestock and Improvement forms, do not appear to match seasonal changes. Dependence on riparian exotic buffel grass pastures accentuates this problem because these pastures in favourable seasons support stock numbers well above what can be supported by
surrounding native vegetation. Until this is accepted, overgrazing of remnant fertile patches will continue and their value as resource capture mechanisms will eventually be lost.
5 Conclusions
The record December 2010 rainfall event was an extreme event exceeding the previous rainfall monthly record for December at Gascoyne Junction by about threefold. The December 2010 flood was also a record event exceeding previous floods by about
0.1 metres. However, the characteristics of the hydrograph were similar to previous major floods since the 1960s, suggesting the properties of the catchment have not changed in this time.
Erosion from the December 2010 flood is likely to have been much greater than the
January and February 2011 floods or the January 2009 flood because an extended dry period preceded the 2010 flood. Examples of erosion described in Section 3 coupled with data from long-term monitoring sites and earlier published reports indicate that accelerated erosion has been occurring in the catchment at least since the 1960s.
At the time of the December 2010 flood the catchment was in poor condition especially between the central Gascoyne and Lyons rivers where the December rainfall event was centred. The condition of the Gascoyne River catchment is poor and has deteriorated since at least the 1930s. High soil infiltration rates are associated with patches of vegetation but these areas represent only a small proportion of the catchment. The catchment will continue to be a high water-shedding environment, whilst the majority of the catchment is dominated by sparsely vegetated areas with low infiltration rates.
Landscape function has deteriorated with the decline in plant numbers. In conjunction, the reduction in carrying capacity of the native perennial pastures through grazing pressure has implications for management in terms of setting appropriate stocking rates to manage any further decline of desirable plants, and therefore landscape function.
Due to the magnitude of the December 2010 rainfall event, coupled with large
watershedding areas with relatively low infiltration rates, a major flood event would likely have occurred irrespective of catchment condition. Major floods can be expected in the future, with subsequent downstream consequences. This is especially likely due to Carnarvon and the horticultural area being situated on a river floodplain and levee built up by successive flood events and the climate being such that tropical cyclones can occur in summer when groundcover is naturally at its minimal.
Whilst it is not possible from the current analysis to assess the effect of catchment
condition on the magnitude of the December 2010 flood, it is likely that were the catchment in better condition (more vegetative groundcover) soil loss would be reduced, particularly away from the major river channels. Improved catchment condition will reduce soil loss from minor and moderate flood events.
Based on the historical and recent review of the Gascoyne River catchment it is likely that future high rainfall events will continue to result in localised flooding, soil loss and damage to infrastructure unless catchment condition is improved.
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